In the known methods for preparing high molecular polyester or polyamide, granulated or pulverized material is crystallized in a first step, with the material being held in a continuous motion in order to prevent the granules from sticking together, which may be done in asymmetric moved mixers, shaft reactors containing stirrers or, as a preferred procedure, in a fluidized bed. The EP-A2-379 684 discloses a particularly advantageous arrangement, in which the material to be treated is made to pass through two fluidized beds arranged in series, the first of which being a whirling fluidized bed having mixing characteristics and the second being a fluidized bed having plug flow characteristics. By using this arrangement, a troublefree crystallization even of a sticky material, by way of example, will be made possible, and there will be obtained a granular material free of any agglomerates and of a uniform degree of crystallization.
After the precrystallizing procedure the material is introduced into a preheating zone, in which the material is heated up to a temperature of 220.degree. C. to 230.degree. C., by way of example, with preferably inert gas flowing through the material. The residence time in the preheating zone amounts to 2 to 4 hours, by way of example.
Following its stay in the preheating zone, the heated material is polymerized in a reactor to the extent of the desired degree, using a further heat treatment in the presence of inert gas. Byproducts, such as aldehyde, carboxylic end groups, etc., are liberated during this final procedure step and carried along with the inert gas. Thereafter, the material is cooled in a cooler in order to make it fit for storage. The typical residence time in the reactor is up to 24 hours.
Depending on the further use of the material, the requirements which the material has to meet will vary. For example, for its use in the foodstuff packing industry it is of importance that the polymer is free of acetaldehyde, which--even in small concentrations--has a specific smell of its own. For other applications, a particularly high mechanical strength or high transparency of the final product may be relevant. These differing requirements call for courses of procedure chosen according to the respective conditions. Thus, high degrees of polymerization can be achieved by using high reaction temperatures and longer reaction times, with, possibly, undesirably large amounts of acetaldehyde being simultaneously formed due to thermic or thermooxidative degradation. In addition, the final product will be discolored yellowish in an unsatisfactory manner in many cases.
In different methods attempts have been made to overcome these problems in all of these cases, which consisted essentially in determining the processing conditions for obtaining the desired product on the basis of serial tests. Thus, the DE-A1-28 34 162 describes such a method in which the contents of acetaldehyde of linear polyethyleneterephthalate is reduced to less than 2.5 ppm. By using this method, the contents of acetaldehyde of samples taken are examined chromatographically, the appertaining, and therefore characteristic, procedure values for temperature and residence time are established on the basis of serial tests, serving as procedure instructions.
A parameter for the degree of polymerization of thermoplastic synthetics is the intrinsic viscosity which conforming to standards is determined by 0.02 to 1.0% solutions, for example in a mixture of phenoltetrachloroethane. The intrinsic viscosity increases with the degree of polymerization. However, this intrinsic viscosity value, which is also called IV, indicates comparable reference values merely for the respective product group, but these values are of great relevance in determining the processing behavior and the application properties to be expected. Thus, determing the IV enables one to establish the degree of polymerization achieved by a certain material. This is an often chosen method in which the temperature and residence time--and, if necessary, other processing conditions as well--are determined in the reactor or the preheating zone according to the IV of the material fed into the reactor or into the preheating zone and are then taken as a basis for the various methods used. Such methods are known, e.g., from the DE-B2-25 59 290, the DE-OS-1 770 410 and also from the above-mentioned DE-A1-28 34 162.
Such methods which are carried out in accordance with the experimental values gained by serial tests, can result in uniform final products only under completely constant conditions. Each deviation, even being small, from the conditions determining the respective procedure instructions, such as temperatures, residence times in the crystallizing part, preheating zone or reactor, amount and nature of the material to be treated, feed conditions between the individual procedure steps, can affect the quality of the final product obtained in an unforeseeable manner, with no possibility existing of any direct influencing during the process itself.